This invention relates to a Coriolis mass flow/density meter for media flowing through a pipe and to a method of generating a measured value representative of mass flow rate.
In Coriolis mass flow/density meters for media flowing through a pipe, the measurement of the mass flow rate is based on the principle of causing a medium to flow through a flow tube inserted in the pipe and vibrating during operation, whereby the medium is subjected to Coriolis forces. The latter cause inlet-side and outlet-side portions of the flow tube to vibrate out of phase with respect to each other. The magnitude of these phase differences is a measure of the mass flow rate. The vibrations of the flow tube are therefore sensed by means of two vibration sensors positioned at a given distance from each other along the flow tube, and converted by these sensors into measurement signals from whose phase difference the mass flow rate is derived.
U.S. Pat. No. 4,187,721 discloses a Coriolis mass flow meter comprising:
a single U-shaped flow tube having an inlet-side end and an outlet-side end, through which flow tube a medium flows during operation;
a support means fixed to an inlet-side end and an outlet-side end of the flow tube such that the flow tube is capable of being vibrated;
a vibration exciter which sets the flow tube into vibration during operation;
a first measuring means positioned on the inlet-side of the flow tube for measuring the vibrations and for delivering a first measurement signal during operation;
a second measuring means positioned on the outlet-side of the flow tube for measuring the vibrations and for delivering a second measurement signal during operation; and
evaluation electronics for delivering, during operation,
a first measured value representative of mass flow rate which is derived from the first and second measurement signals.
Further EP-A 849 568 (corresponding to U.S. Ser. No. 08/940,644, filed Sep. 30, 1997) discloses a Coriolis mass flow meter comprising:
a single straight flow tube having an inlet-side end and an outlet-side end, through which flow tube a medium flows during operation;
a support means fixed to an inlet-side end and an outlet-side end of the flow tube such that the flow tube is capable of being vibrated;
a vibration exciter which sets the flow tube into vibration during operation;
a first measuring means positioned on the inlet-side of the flow tube for measuring the vibrations and for delivering a first measurement signal during operation;
a second measuring means positioned on the outlet-side of the flow tube for measuring the vibrations and for delivering a second measurement signal during operation; and
evaluation electronics for delivering, during operation,
a measured value representative of mass flow rate which is derived from the first and second measurement signals.
In addition each of U.S. Pat. Nos. 4,660,421 and 4,733,569 discloses a Coriolis mass flow meter comprising:
a spiraled flow tube having an inlet-side end and an outlet-side end, through which flow tube a medium flows during operation;
a support means fixed to an inlet-side end and an outlet-side end of the flow tube such that the flow tube is capable of being vibrated;
a vibration exciter which sets the flow tube into vibration during operation;
a first measuring means positioned on the inlet-side of the flow tube for measuring the vibrations and for delivering a first measurement signal during operation;
a second measuring means positioned on the outlet-side of the flow tube for measuring the vibrations and for delivering a second measurement signal during operation; and
evaluation electronics for delivering, during operation,
a measured value representative of mass flow rate which is derived from the first and second measurement signals.
Furthermore each of U.S. Pat. Nos. 4,491,025, 4,660,421 and 5,218,873 discloses a Coriolis mass flow meter with two communicating flow tubes through which a medium flows during operation. These flow tubes are interconnected by means of an inlet-side first manifold having an inlet-side first end and an outlet-side second manifold having an outlet-side second end and are fixed by a support means such that the flow tube is capable of being vibrated.
Both U.S. Pat. No. 4,187,721, which was referred to at the beginning, and EP-A 849 568 mention that Coriolis mass flow meters can also be used to measure the instantaneous density of the flowing medium. For the invention it is therefore assumed that the devices referred to above as Coriolis mass flow meters also measure the instantaneous density of the flowing medium even though this is not always described in the individual documents, since it is self-evident.
In Coriolis mass flow meters and Coriolis mass flow/density meters, the ratio of the width D of the flow tube to the length L of the flow tube (D/L ratio) is of significance for the measurement accuracy. If a single flow tube is used, the width D is virtually equal to the nominal diameter of the pipe.
At a D/L ratio greater than approximately 0.05, the instantaneous velocity field of the medium in the flow tube may affect the measurement accuracy so that the resulting increased measurement error may no longer be negligibly small. Measurements have shown that at D/L ratios greater than 0.05, this influence of the velocity field may cause an additional error of a few per mille to one percent.
However, the minimization of the D/L ratio is limited by constraints placed on the design of the meter, namely, on the one hand, by the nominal pipe diameter specified in a concrete application and, on the other hand, by the fact that meters are required which are as short and compact as possible.
It is an object of the invention to provide a Coriolis mass flow/density meter which provides highly accurate measurement results independently of the instantaneous velocity field while being as compact in construction as possible. Another object is to provide a method of producing such measurement results.
To attain the first-mentioned object, the invention provides a Coriolis mass flow/density meter for a medium flowing through a pipe, said Coriolis mass flow/density meter comprising:
at least one flow tube having an inlet-side end and an outlet-side end, through which at least one flow tube the medium flows during operation;
a support means fixed to an inlet-side end and an outlet-side end of the flow tube such that the flow tube is capable of being vibrated;
a vibration exciter which sets the flow tube into vibration during operation;
a first measuring means positioned on the inlet-side of the flow tube for measuring the vibrations and for delivering a first measurement signal during operation;
a second measuring means positioned on the outlet-side of the flow tube for measuring the vibrations and for delivering a second measurement signal during operation;
a third measuring means for delivering a third measurement signal during operation which is representative of the instantaneous Reynolds number of the flowing medium; and
evaluation electronics for delivering, during operation,
a first measured value representative of mass flow rate which is derived from the first, second, and third measurement signals, and
a second measured value representative of the instantaneous density of the medium, which is derived from the first and second measurement signals.
Furthermore, the invention consists in a method of generating a first measured value representative of mass flow rate by means of a Coriolis mass flow/density meter for a medium flowing through a pipe, said Coriolis mass flow/density meter comprising:
at least one flow tube having an inlet-side end and an outlet-side end, through which at least one flow tube the medium flows during operation;
a support means fixed to an inlet-side end and an outlet-side of the flow tube, such that the flow tube is capable of being vibrated;
a vibration exciter which sets the flow tube into vibration during operation,
said method comprising the steps of:
sensing the vibrations of the flow tube and generating a first measurement signal representative of inlet-side vibrations and a second measurement signal representative of outlet-side vibrations for developing an intermediate value representative of an uncorrected mass flow rate;
generating a third measurement signal representative of an Reynolds number of the flowing medium by means of the intermediate value and by means of a fourth measurement signal representative of a dynamic viscosity of the medium; and
correcting the intermediate value by means of a correction value derived from the third measurement signal.
In a first embodiment of the Coriolis mass flow/density meter according to the invention, the evaluation electronics provide a correction value derived from the third measurement signal.
In a second embodiment of the Coriolis mass flow/density meter according to the invention, the evaluation electronics provide the correction value by means of a constant correction value for laminar flow determined by calibration, by means of a constant correction value for turbulent flow determined by calibration, and by means of an interpolated correction value determined according to an interpolation function lying between the two constant correction values.
In a third embodiment of the Coriolis mass flow/density meter according to the invention, the evaluation electronics comprise a table memory in which Reynolds-number-dependent digitized correction values are stored, and which provides the correction value by means of a digital memory access address formed on the basis of the third measurement signal.
In a fourth embodiment of the Coriolis mass flow/density meter according to the invention, the evaluation electronics provide an intermediate value derived from the first and second measurement signals which is representative of an uncorrected mass flow rate.
In a fifth embodiment of the Coriolis mass flow/density meter according to the invention, the evaluation electronics deliver the first measured value in response to the intermediate value and the correction value.
In a sixth embodiment of the Coriolis mass flow/density meter according to the invention, the Coriolis mass flow/density meter comprises a fourth measuring means which measures a dynamic viscosity of the medium and delivers a fourth measurement signal representative of said dynamic viscosity.
In a seventh embodiment of the Coriolis mass flow/density meter according to the invention, the third measuring means delivers the third measurement signal in response to the uncorrected intermediate value and the fourth measurement signal.
In an eighth embodiment of the Coriolis mass flow/density meter according to the invention, the fourth measuring means measures a kinematic viscosity of the medium and delivers a fifth measurement signal representative of said kinematic viscosity.
In a ninth embodiment of the Coriolis mass flow/density meter according to the invention, the fourth measuring means delivers the fourth measurement signal in response to the second measured value and the fifth measurement signal.
In a tenth embodiment of the Coriolis mass flow/density meter according to the invention, the vibration exciter comprises a coil which is supplied with excitation energy and from whose current and/or voltage the fourth measuring means derives the fourth measurement signal and/or the fifth measurement signal.
In an eleventh embodiment of the Coriolis mass flow/density meter according to the invention, the fourth measuring means derives the fourth measurement signal and/or the fifth measurement signal from a pressure difference measured along the pipe.
In a first embodiment of the method according to the invention, the fourth measurement signal is derived from a current and/or a voltage of an excitation energy supplied to the vibration exciter.
In a second embodiment of the method according to the invention, the fourth measurement signal is derived from a pressure difference measured along the pipe.
One advantage of the invention is that even at a D/L ratio greater than 0.05, the Coriolis mass flow/density meter provides a mass flow value in which the effect of the instantaneous velocity field on measurement accuracy has been compensated.